Applicant""s invention relates generally to radiocommunication systems, e.g., cellular or satellite systems and, more particularly, to techniques for supporting and enhancing emergency calling procedures in such systems.
The growth of commercial radiocommunications and, in particular, the explosive growth of cellular radiotelephone systems have changed the ways in which people communicate. One survey indicates that about 80% of the people who purchase mobile communication units and service subscriptions do so to enhance their personal security. Presumably, many of these subscribers would expect to use their mobile units to aid them in urgent situations, e.g., when their vehicle has become disabled or in an emergency situation requiring rapid medical and/or police response. In these circumstances it would be desirable that access to the radiocommunication system be as fast as possible so that an emergency service center can respond quickly to the user""s request for assistance.
Consider, solely for the purposes of example, the prevalent digital cellular radiotelephone system in North America known as the digital advanced mobile phone service (D-AMPS), some of the characteristics of which are specified in the interim standard IS-54B, xe2x80x9cDual-Mode Mobile Station-Base Station Compatibility Standardxe2x80x9d, published by the Electronic Industries Association and Telecommunications Industry Association (EIA/TIA). D-AMPS is a TDMA cellular radiotelephone system wherein each radio channel is divided into a series of time slots, each of which contains a burst of information from a data source, e.g., a digitally encoded portion of a voice conversation. The time slots are grouped into successive TDMA frames having a predetermined duration. According to IS-54B, each TDMA frame consists of six consecutive time slots and has a duration of 40 milliseconds (msec). Thus, each frame can carry from one to six channels (e.g., one to six radio connections).
Because of a large existing consumer base of equipment operating only in the analog domain with frequency-division multiple access (FDMA), IS-54B is a dual-mode (analog and digital) standard, providing for analog compatibility in tandem with digital communication capability. For example, the IS-54B standard provides for both FDMA analog voice channels (AVCs) and TDMA digital traffic channels (DTCs), and the system operator can dynamically replace one type with the other to accommodate fluctuating traffic patterns among analog and digital users. The AVCs and DTCs are implemented by frequency modulating radio carrier signals, which have frequencies near 800 megahertz (MHz) such that each radio channel has a spectral width of 30 kilohertz (KHz).
The IS-54-B standard also provides for a number of analog control channels (ACC) on which system accesses can be initiated and system overhead information can be communicated to the mobile units. A subsequent standard, referred to as IS-136, adds specifications for digital control channels (DCCs).
Conventional access procedures calls vary from system to system but typically include the following basic steps. First, when a mobile unit is turned on, it locates a control or access channel over which it can receive paging messages and other overhead information and over which it can send messages to initiate a call. As radiocommunication systems grow more complex, e.g., due to the increasing customer base and limited bandwidth available to provide additional capacity, a greater number of control channels tend to be operative in any given geographic area. Accordingly, systems (such as those defined by IS-54B and IS-136) have established criteria which define the location and selection process that a mobile unit will follow when locking onto a control channel. These procedures, however, may result in a mobile station finding a first control channel but not locking to that control channel since it fails to meet the selection criteria and then searching for and evaluating one or more additional control channels before a suitable candidate is found.
Additional delay in accessing the system may be associated with the signaling defined by these various systems for originating a call from the mobile unit, i.e., the process of acquiring a traffic channel. When a mobile subscriber initiates a call, e.g., by dialing a telephone number and pressing the xe2x80x9csendxe2x80x9d button on the mobile unit, the mobile unit transmits the dialed number and its mobile identity number (MIN) and an electronic serial number (ESN) over the control channel to the base station. The ESN is a factory-set, xe2x80x9cunchangeablexe2x80x9d number designed to protect against the unauthorized use of the mobile station. The base station forwards the received numbers to the MSC, which validates the mobile station, selects an AVC or DTC, and establishes a through-connection for the call as described above. However, the mobile unit may also be required to send other messages, e.g., an authentication message, as part of the call origination process. These additional messages introduce further delay in the call set-up process since an access attempt is not deemed successful until all messages are transmitted, which delay may be an acceptable trade-off for non-emergency calls given the benefits (e.g., reduction in fraud) associated therewith, but which may be unacceptable for emergency system access.
Moreover, each message required by the system for call set-up requires additional bursts to be transmitted by the mobile unit and correctly received by the base station. Since the various mobile units attempt system access using control channels on a contention basis, sometimes two mobile unit""s bursts will collide on a same sub-portion of a control channel. Thus, radiocommunication systems provide feedback to the mobile units regarding the success or failure of their transmitted access bursts. This information is used by the mobile units to determine if retransmission of a particular burst or bursts is necessary, which retransmission procedure is also predefined by the system and which may introduce still further delay in the assignment of a traffic channel and subsequent relaying of the emergency call from the mobile unit to the base station.
Accordingly, it would be desirable to provide techniques whereby access to radiocommunication systems could be accelerated for emergency calls placed by mobile units.
According to exemplary embodiments of the present invention, various techniques are described to accelerate a mobile unit""s access to a radiocommunication system when the mobile unit is placing an emergency call. For example, according to one exemplary embodiment, the process by which a mobile station locks to a control channel may be expedited by ignoring certain criteria which are normally used to evaluate control channels when a mobile unit is powered-on and an emergency call is placed.
According to another exemplary embodiment, emergency access may be expedited by reducing the number of messages which are transmitted by the mobile unit as part of the call origination process. For example, an authentication message may be omitted for emergency access since it may be desirable to process a request for emergency assistance regardless of the authenticity of the equipment or validity of the subscription by which the emergency call is placed. By transmitting less data during emergency call accesses, faster and more reliable system access is achieved.
According to a still further exemplary embodiment of the present invention, retransmission rules may be adjusted or ignored by a mobile unit making an emergency call access. For example, a mobile unit may continuously retry erroneously received transmission bursts, rather than waiting for a predetermined time in accordance with conventional retransmission rules, to accelerate system access.